Large-diameter cylinder-shuttle air guns for producing powerful impulses are known to those skilled in the art. One such design is disclosed in U.S. Pat. No. 5,432,757 to Chelminski which is incorporated fully herein be reference. Such large-diameter cylinder-shuttle air guns have many advantages which result from the fact that a powerful impulse of air may be released is readily repeatable as desired. Such powerful impulses are useful for seismic exploration.
For example, in seismic exploration, such air guns are used to generate seismic impulses transmitted into the medium in which the air guns are positioned, such as in (water) the sea, or in a more solid mass which contains water, (such as drilling mud in oil wells). It is also possible to submerge such seismic air guns in marshland, swamp or mud areas that are infused with sufficient water that the seismic surveying apparatus described herein can be used. Accordingly, the term “water” as used herein is intended to include marshland, swamp, mud or other medium that contains sufficient water to enable such apparatus to be used.
In seismic surveying, seismic wave energy is generated by air guns submerged in the water and is utilized to investigate subsurface geological conditions and formations. For this purpose, one or more of such air guns (typically one or more arrays of, for example, four to forty air guns) are submerged in the water, and compressed air or other gas(es) under pressure is fed to the submerged guns and temporarily stored therein. At the desired instant, the air guns are actuated, i.e., fired, by means of a trigger mechanism (such as an electrically operated trigger valve i.e. a solenoid valve), and the pressurized gas is abruptly released into the surrounding water. In this manner, powerful seismic waves are generated capable of penetrating deeply into subsurface material to be reflected and refracted therein by the various strata and formations. The reflected or refracted waves are recorded and analyzed to provide information and data about the geological conditions and formations reflecting the seismic waves.
While the known air guns are fairly effective at producing the necessary seismic waves, they suffer from several disadvantages. When a seismic gun fires, the seismic pulses generated 200, FIG. 1, include a first, primary and generally large pulse 186 followed by several smaller secondary pulses 188 (only one of which is shown) which get progressively smaller as a function of time. This pattern is caused, in part, by the movement of the firing flange which opens communication between the annular firing chamber and one or more ports in the air gun through which a sudden blast of compressed air is discharged from the firing chamber out through the ports, thus creating a large-circumference torus bubble which produces a high peak pressure in a surrounding body of water. See U.S. Pat. No. 5,432,757 to Chelminski, for additional information.
Ideally, the pulse generated by the air gun would include only a single pulse 186 having an infinite slope and a large amplitude. The presences of the secondary pulses 188 tend to mask the received or reflected signal created by the primary pulse 186 and therefore must be compensated for and reduced or eliminated as much as possible.
The known large-diameter cylinder-shuttle seismic air guns 100, FIG. 2, include an annular shaped moveable shuttle 102 slidely disposed within a cylindrically shaped body 104. The shuttle 102 includes a operating chamber flange 106 disposed at one end of the shuttle 102 within an operating chamber 108 and a firing chamber flange 110 disposed proximate a firing chamber 112. In the loaded position, the firing chamber flange 110 blocks the firing chamber 112 from the annular port 114 and high pressure air builds-up within the firing chamber 112.
A force then acts on the shuttle 102 to move the shuttle 102 in the direction of arrow 115 from the loaded position to the unloaded position where the annular port 114 is open to the atmosphere, thereby releasing the high-pressure air from the firing chamber 112. For more details on the operation of such a known seismic gun 100, see U.S. Pat. No. 5,432,757 to Chelminski.
One factor that determines the amplitude of the primary pulse 106 (FIG. 1) is the amount of time that the port 114 is open to the atmosphere, i.e., the port time. The port time is determined, in part, by a force Fr acting on the shuttle 102 that returns the shuttle 102 to the loaded position from the unloaded or fired position. This force Fr is generated by the air pressure within the operating chamber 108 acting on the shuttle 102. In particular, the air pressure within the operating chamber 108 acts on shuttle flange 106 and the end 120 of the shuttle 102 (i.e., the thickness 120 of the shuttle 102). Ultimately, the air pressure acting on both sides of shuttle flange 106 cancels itself out and the resulting force causing movement of the shuttle is determined by the cross sectional thickness/area 120 of the shuttle flange 102.
One disadvantage that the known seismic air guns 100 is that the port time is too small because the return force Fr acting on the shuttle 102 is too large. This reduces the amount of pressurized air that escapes from the firing chamber 112 through port 114, thereby reducing the amplitude of the primary pulse 186 (FIG. 1).
Accordingly, what is need is a cylindrical shuttle air gun having a high sound level output with a central bore of large enough diameter to be capable of carrying control lines and air hoses, passing through the bore. The air gun should allow more air to escape, thus creating a larger initial pulse by virtue of creating a larger Torus bubble and thereby minimizing the impact of secondary pulses on the effectiveness of the air gun. The seismic air gun should preferably allow the return force to be optimized to increase the port time and thereby increase the amplitude of the primary wave, while reducing the amplitude of the secondary waves, by allowing the shuttle to stay open longer after the peak of the primary pulse has occurred. This “after-flow” air tends to dampen the strength of the secondary pulses. The seismic air gun must also be strong enough to withstand the forces generated during repeated firing/loading.
It is important to note that the present invention is not intended to be limited to a system or method which must satisfy one or more of any stated objects or features of the invention. It is also important to note that the present invention is not limited to the preferred, exemplary, or primary embodiment(s) described herein. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.